1. Field of the Invention
This invention relates to electric generators and more particularly relates to generators and alternators operating at high efficiency which are suitable for use under a variety of changing conditions.
2. Description of Related Art
There are numerous generators used throughout the world for generating electric power. Such generators use the basic principal of electromagnetic induction to convert the energy of motion into electricity. If an electrical conductor such as a wire is moved through a magnetic field, or conversely if a magnetic field is made to change in the presence of such a conductor, an EMF or voltage will be induced in the conductor. The voltage induced in the conductor is determined by the following factors:                (1) If the conductor is a wire in the form of a coil the greater the number of turns in the coil, the greater will be the induced EMF.        (2) The faster the conductor moves through the magnetic field the greater will be the induced EMF.        (3) The stronger the interacting magnetic field is the greater will be the induced EMF. If the conductor is stationary but the magnetic field changes (such as the case with permanent magnet alternators) the faster the rate of change the greater will be the induced EMF.        
When power is required such as for lighting applications, a connection is made between the producing conductor of the generator and to the device. This causes a current to flow from the generator to the device. Whenever a generator delivers power to some device an associated mechanical drag on the moving parts of the generator results. The more power that is pulled from a generator, the greater will be the mechanical requirements needed to keep the generator producing power. Many generators such as the ones powered by gasoline engines used in portable applications are designed to run at a fixed speed under the conditions of a given load. Such generators work well because the source of mechanical power (the gasoline engine) can be controlled and soon reaches a steady state for any given load. In many cases, this is quite suitable.
There are many generators that are designed to produce power under varying conditions of speed and required power output. A good example of this type of generator is the alternator used to power the electrical systems in automobiles. The power output of such alternators must be carefully controlled to maintain proper battery life. If not enough power is supplied to the battery and the associated electrical needs of the car, the battery will run down. If too much power is delivered to the battery, the battery will overcharge resulting in reduced battery life, and possibly over voltages which can damage certain electrical components in the electrical system of the automobile. In addition to the changing needs of the automobiles electrical system, the speed of the engine is always changing. Sometimes the engine is slowly idling at a few hundred RPM. Other times, the engine is running at several thousand RPM. Because of this, alternators used for the generation of power in automobiles have electrical circuitry which regulates the output power of the alternator to the needs of the electrical system. This is accomplished by employing two sets of electromagnets. One set is located into the rotary portion of the alternator or rotor, and the other set of electromagnets is located in the stationary portion of the alternator or stator. The rotor electromagnets require electrical power to produce the initial magnetic field. This power comes from a set of brushes that supply electricity to the commutator of the rotor to deliver power to their windings while at the same time allowing the rotor to rotate. The rotor electromagnet consists of many turns of a light gauge wire such as #21. Because of this, not much current will flow into this electromagnet. This results in a low power demand on the brushes. The stator electromagnet consists of a few turns of heavy gauge wire such as #14. The stator windings produce a substantial amount of AC current due to the changing magnetic field caused by the rotating rotor electromagnet. This AC current is then rectified to DC using diodes. Voltage regulation circuitry is used to control power going into the rotor electromagnet. In this way, a small amount of control current in the rotor results in a very large change in output current and voltage from the stator windings. This configuration works well for electric generators and alternators employing rotating electromagnet windings because a fine control in voltage output is easily attained under constant or variable speed conditions.
Numerous alternators have been built employing permanent magnets in the rotor, and electromagnets in the stator. These alternators can be used to generate AC power, or alternatively can have their power rectified with diodes to produce DC power. Such alternators are inherently more reliable because of their brushless design. They also have fewer moving parts to wear out and do not require input power to provide output power. These brushless permanent magnet alternators also produce less heat owing to the fact that there are no rotor windings. Such alternators have been employed in motorcycles and other lightweight vehicles. For example, The Ducati SL500 Pantah alternator is used in many Ducati motorcycles. This particular alternator is a permanent magnet alternator employing a rotor having permanent magnets surrounded by a stator electromagnet assembly. This permanent magnet alternator is designed to be used with a specific regulator, the SL500 Pantah regulator which rectifies the AC power from the alternator as well as regulating the output power. Many motorcycles utilize an external rotor which doubles as the flywheel. As usual, voltage regulation is carried out using electrical circuitry which is placed between the output from the alternator and the battery. Such an approach while being rather simple, cost effective, and straightforward has its drawbacks. The alternator output voltage to the regulating circuitry is dependent on rotor speed. Because of this the power output from such alternators is rarely occurring under optimum conditions of efficiency. When operating at low RPM values such permanent magnet alternators must have enough windings to provide sufficient voltage for the electrical system. Such windings often have relatively high electrical resistance owing to the need to use small gauge wire to fit many turns of wire on the stator electromagnet. Conversely, in order to deliver substantial current at high RPM conditions without excessive voltage, the wire diameter needs to be of a relatively large gauge with only few turns needed. In practice a compromise in performance on either end of the RPM scale is reached by choosing an intermediate gauge wire diameter having a substantial number of turns. Such systems have a difficult time delivering the needed power under the wide ranges of RPM values normally experienced during use. Despite these drawbacks existing electrical systems employing permanent magnet alternators have a proven record of reliability for use in motorcycles and other lightweight vehicles. Further improvements in efficiency and reliability of systems employing these permanent magnet alternators can be expected by the use of improved power control circuitry.
In addition to alternators for vehicle use permanent magnet alternators are being increasingly employed in generators used in standby power applications as well as generators used in alternative energy systems utilizing forms of power such as wind and hydroelectric. One example of such a system is outlined in U.S. Pat. No. 4,720,640. In this patent, a turbine is driven by a fluid such as moving air or moving water. On the periphery of this turbine are located permanent magnets. A stator consisting of multiple electromagnets is located around the outside periphery of the rotor permanent magnets with the electromagnet pole faces located in close coupling proximity to the pole faces of the rotor permanent magnets. A similar system is outlined in U.S. Pat. No. 5,696,419 to Rakestraw. As in U.S. Pat. No. 4,720,640, a fluid driven impeller having a periphery of permanent magnets is employed as the rotor. The stator electromagnets are C-Shaped and straddle the permanent magnet pole faces in the rotor. One advantage this system offers for some applications is a power curve that tends to be self-limiting under the conditions of high RPM. Despite these and other numerous advances in generators and alternators there is a need for an alternator or a generator having variable power output windings employing electrical circuitry which will automatically vary the number of these windings based on running RPM and needed power demands. In this way the power output of the generator itself can compensate for changes in RPM and load requirements with minimal or no voltage regulation circuitry required.
Power generating windmills are a prime example. Wind speeds are always changing, however, the output voltage during power delivery needs to be relatively constant. In low wind conditions the ideal power generating electromagnet windings are preferably large in number to generate the needed voltage to power these energy systems. In addition the generator rotor drag caused by the output power has to be somewhat limited to prevent the stalling of the generator. Because of this many turns of a relatively small gauge electromagnet wire would be desired. During high wind conditions a few turns of a relatively heavy gauge wire would be the desire. In this way enough current at a suitable voltage can be delivered to make use of the power available under high wind conditions, not overheat from too high a winding resistance, and be able to create enough mechanical drag to keep the windmill impeller from spinning too fast and flying apart.
Other examples include alternators for use in vehicles such as automobiles, motorcycles, trucks, and the generation of electric power from variable speed sources and/or for variable power requirements.
In view of the foregoing, it is an object of this invention to provide an electric generator or alternator which eliminates the need for armature brushes.
It is a further object of this invention to provide a generator or alternator having a high efficiency under the conditions of changing rotor speed.
It is a further object of this invention to provide a generator or alternator having variable output voltage characteristics.
Finally, it is an object of this invention to reduce the requirements of power control circuitry normally used to modify generator output voltage into a useable form.